FCC processes are well known. In the more usual FCC processes employing riser reactors the catalyst and the hydrocarbon feed flow upward, while in FCC processes employing downflow reactors the catalyst and the hydrocarbon feed flow downward.
In riser reactors, solids flow upward due to the lift caused by the ascending vaporized feed. However, the velocity of the hydrocarbon vapor is lower near the wall than it is near the center of the reactor. Therefore, the catalyst will move more slowly near the reactor wall than near the center, resulting in a slower moving area with a high catalyst density near the wall and a low-resistance path of ascending hydrocarbon vapor near the center. Hence, the hydrocarbon vapor mainly flows through the center, whereas the catalyst is mainly located near the walls. The resulting flow pattern is called core-annulus.
Furthermore, the upward flow of solid catalyst and hydrocarbon vapor in riser reactors oppose gravity, resulting in a catalyst flow that is significantly slower than the much lighter hydrocarbon flow. The ratio of feed velocity to catalyst velocity, i.e., the slip factor, generally is about 2–3. This results in backmixing of the catalyst, leading to longer residence times of the catalyst and the occurrence of undesirable secondary reactions such as overcracking.
In contrast to riser reactors, downflow reactors do not display large differences in velocity and catalyst density between the center and the wall of the reactor. Furthermore, as the catalyst particles do not oppose gravity, the difference in velocity between the catalyst flow and the hydrocarbon flow in these reactors is smaller than in riser reactors. The slip factor of downflow reactors generally is about 1.
Consequently, backmixing is largely avoided, the catalyst is more evenly distributed across the entire reactor, and the effective contact time of the catalyst and the feed in a downflow reactor is less than in a riser reactor.
U.S. Pat. No. 5,498,326 teaches a process for catalytic cracking and the associated apparatus in which the cracking reaction takes place in two substantially vertical and successive reaction zones, the feed being introduced into the first zone where it circulates from the top downwards, then at least a portion of the product obtained is introduced in a second reaction zone in which it circulates in an ascending fashion. It is alleged that in downward reactors the separation is made more difficult in virtue of the substantial concentration of catalyst in the reaction medium, which requires specifically designed equipment if suitable efficiency is to be enjoyed. That is why the separation is effected after the reaction in the ascending mode. However, this arrangement implies that the contact time between hydrocarbon vapor and catalyst is made excessively long, with the ensuing well-known drawbacks resulting from long contact time, that is, thermal cracking, coke, etc. Thus, the situation depicted in FIG. 2 of this patent shows that after the reaction in downflow reactor 27, in intermediate area 34 the reaction between hydrocarbon vapor and catalyst continues for a certain period of time before the separation of hydrocarbon vapor and catalyst designed to occur in riser 35. Therefore the advantages from the use of both downflow and riser reactors may be wasted and the efficacy of the technique to secure the alleged benefits are rather low.
U.S. Pat. No. 5,468,369 ('369 patent) teaches a process for short contact time fluidized catalytic cracking of heavy oil feed using a reactor with an upflow catalyst-to-oil vaporizer and a downflow reactor. Catalyst slip permits efficient mixing and limited conversion in the upflow section, while cracking is completed in the downflow reactor with minimal segregation of catalyst. The catalyst has a 25 wt % Y zeolite content and total vapor residence time is less than 5 seconds. The patent contains a thorough explanation on the kinds of short contact time reactors, that is, risers and downflow reactors. It is stated that by virtue of the nearly absence of slip in downflow reactors, no core-annular or similar flow forms, the phases retaining throughout the entire reactor exactly the state of admixture which they have upon leaving the mixing section at the top. The solids do not migrate, stall, scatter or clump. For this reason downflow reactors cannot induce suboptimal kinetics due to radial phase separation.
Also discussed in said US '369 patent is a shortcoming of the downflow reactor, that is, initial catalyst-to-oil mixing can be difficult to achieve because gravity acts to decrease the solids density in the zone of initial contacting, contrary to the effects noted in a riser. Entering solids tend to drop immediately downward and cannot reflux or circulate in any way, which might enhance initial heat and mass transfer with the oil. If the inlet flows of catalyst and oil were perfectly steady and perfectly balanced, this effect would be less important. However fluidized solids flow is never truly steady and usually features small-amplitude oscillations in solids density whether moving in a riser or a downflow reactor. Thus the gas-solids mixture in a cracking reactor actually consists of alternating catalyst-rich and catalyst-lean pulses, the variations being fairly small. In a riser the pulses have little impact because oil vapor, traveling faster than catalyst, passes through the regions of high and low solids density and experiences on balance the proper (average) catalyst-to-oil ratio. A downflow reactor, by contrast, has no mechanism for moving oil from catalyst-lean to catalyst-rich regions and vice versa. Oil, which enters in a region of low solid density will remain there through the entire reactor, ultimately subject to thermal cracking due to premature deactivation of the catalyst. Oil in a high-density region tends to be overcracked. Hence the products of low catalyst-to-oil and high catalyst-to-oil ratios do not in any way “average” to the products of the correct catalyst-to-oil ratio. As will be seen later in the present specification, the present invention overcomes this and other drawbacks of downflow reactors by using a novel regenerated catalyst distributor, which improves the efficiency of the catalyst-to-oil mixing, a proprietary feed injectors and a lean catalyst phase.
Further, it should be pointed out that the '369 patent fails to achieve the alleged goals since the upflow internal mixing section as illustrated in FIG. 2 of said patent suffers from the following drawbacks: i) the feed is injected in 115 according to the axial mode, which is an out-of-date mode, implying a poor feed distribution, ii) there is a high amount of catalyst in reactor 100, on the other hand, there is a gap between the moment the feed enters reactor 100 and it vaporizes, as a consequence, a great possibility of coking exists; iii) another point to be considered in reactor 100, is an excessive contact time between catalyst and feed, with the consequent increase in coke build-up on the catalyst.
Generally, it is considered that the two main design features in FCC processes are the catalyst-to-oil mixing at the feed injection point and the separation of hydrocarbon products from spent catalyst.
As regards the catalyst-to-oil mixing, important state-of-the-art documents are cited below.
As for the catalyst-to-oil mixing the present invention makes use of a proprietary device, object of Brazilian Patent Application PI BR 0101433-1 herein completely incorporated as reference, said device being a portion of the regenerated catalyst distributor of the invention, said device basically comprising, according to FIG. 1 of said Brazilian application, a conduit (1) the diameter of which is smaller than the diameter of the downflow reactor, a plate (3) provided with evenly distributed perforations and which makes up the bottom of a collector/distributor catalyst vessel (2) predominantly cylindrical, axially and longitudinally mounted inside conduit (1). As will be seen later in the present specification, said device is extremely useful in achieving an optimized catalyst distribution to the downflow reaction section immediately below.
However, the device taught in PI BR 0101433-1 does not allow processing a mixture of catalyst and gas. Catalyst only may be adequately distributed through the perforated plate, for instance, received from a slide valve mounted on top of the perforated plate. This restricts the use of such a device to a situation where there is catalyst flow only, preferably below a regenerated catalyst slide valve.
As opposed to the limitations of said device, the present application can be used to an admixture of catalyst and carrying fluid from a riser, then properly separating carrying fluid from catalyst and adequately distributing catalyst in a distributor to contact the feed so as to vaporize and crack same.
Another aspect of the catalyst-to-oil mixing is the feed vaporization. As regards this point, the present invention makes use in the present downflow reactor apparatus, of another proprietary device, fully incorporated herein as reference, which is a feed injector, published as WO 01/44406A1, such injector comprising two concentric conduits, where an atomization fluid flows through an inner conduit, while the liquid feed flows through the annular space formed by the outer surface of the inner conduit and the inner surface of the outer conduit; an atomization unit having nozzles arranged in rows, with one row having central nozzles connected to the inner conduit for atomization fluid, and two or more rows of side nozzles, connected to the outer feed conduit, the central nozzles and side nozzles of the atomization unit being geometrically placed so that energy of the atomization fluid is fully transferred by contact to the flow of feed, this resulting in the complete atomization of the feed; and a mixing chamber formed by the edges of the central nozzles, the dimensions of which are able to prevent the coalescence of the formed oil droplets.
Feed injector claimed in WO 01/444064A1 is a highly efficient injector. The combined use of such a feed injector or similarly efficient injectors with characteristics of the downflow technology herein disclosed overcomes the draw backs typical of downflow reactors due to the low slip factor which causes an effective short contact time of catalyst within the mixing zone. The quick feed vaporization secured by such efficient injectors allows the cracking reaction to proceed in those short contact times.
The second important feature in FCC processes, v.i.z., separation of products and spent catalyst after the cracking reaction, is considered in two patents: U.S. Pat. No. 4,514,285 and U.S. Pat. No. 5,582,712.
Quick separation of hydrocarbon products and catalyst is a mandatory consequence of the short contact reaction time achieved in a downflow reactor, since it is nearly meaningless to profit from the benefits of the downflow reactor while such benefits may be lost by a too long separation step.
U.S. Pat. No. 4,514,285 teaches a catalyst separation method for separation of spent catalyst from reaction products and for separation of regenerated catalyst from flue gases. The separation method is referred to in said patent as a ballistic system, that is, a system in which the momentum of the catalyst particles assisted by gravity is utilized to effect separation of catalyst from reaction products or flue gases without the need for complex equipment. The proposed equipment involves a ballistic separation zone having a cross-sectional area within the range of 20 to 30 times the cross-sectional area of the reaction zone and open at its lower end to a stripping zone therefore permitting unobstructed free fall of catalyst under the influence of gravity. The problem is that discharging catalyst into separation section 15 of vessel 16 having such a high ratio of cross sectional area implies in augmented possibilities of undesirable thermal cracking and coke build-up.
Also, the concept of U.S. Pat. No. 4,514,285 does not allow to processing a mixture of catalyst and gas. Catalyst only may be adequately distributed through the perforated plate, for instance, catalyst received from a slide valve mounted on the top of the perforated plate.
U.S. Pat. No. 5,582,712 is also directed to quick separation methods involving two-step separation of spent catalyst and hydrocarbon products resulting from a downflow FCC reactor. It should be pointed out that the use of enclosing cyclone separators taught by said US patent may be a problem whenever there is any unsteady operation of the reactor, which leads to a lower separator efficiency and therefore to undesirable overcracking reactions due to entrainment of the gas phase which reacted with the catalyst suspension, as well as heavy catalyst losses to the product fractioning system and auxiliary equipment thereof.
Also, U.S. Pat. No. 5,582,712 does not recognize the necessity of providing an external path for stripper gases. On the contrary, the stripper gas is withdrawn through a conduit together with the vapor products. Since the kind of conduit taught in said patent is the normal gas outlet of a cyclone, this arrangement will render inoperable a commercial FCC unit due to the reasons pointed out below.
FCC commercial unit cyclones operate at an efficiency of 99.995% plus. Such a high efficiency is of paramount importance to assure the operability of the unit. A typical FCC unit with a capacity of 37,000 BPD circulates 30 ton per minute of catalyst as solid particles through the gas/solid separation device at the end of the transport zone. A device having an efficiency of 99.995% means a daily solids loss of the order of 2.2 tons to downstream equipment. If for any reason the efficiency of a cyclone is slightly reduced, for example, to 99.985%, catalyst loss may attain 6.5 tons. This amount may cause circulation problems due to equipment fouling, loss of heat exchange capacity, pump losses due to rapid erosion, besides an extremely high catalyst operational cost.
So, a device based on cyclone separation without considering an external passage for the stripper gas or for small amounts of gas escaping from the cyclone due to pressure surges would probably suffer from an efficiency loss even much higher than that of the cited example. Lab scale as well as industrial unit tests by the Applicant confirm this assertion.
On the other hand, U.S. Pat. No. 5,569,435 of the Applicant and fully incorporated herein as reference teaches a system for separating catalyst particles from reacted hydrocarbons which includes an unconfined cyclone device made up of a diplegless cyclone opening directly into a large volume separator vessel downwardly through a mouth and upwardly through an annular space between concentric pipes. The proposed system is applied to upward FCC reactors. In the present application said system may be applied to effect the quick separation of spent catalyst from reacted hydrocarbons in a downward reactor.
Therefore, the cited literature teaches methods and systems for improved catalyst-to-oil mixing as well as for spent catalyst/reacted hydrocarbons separation, for upward reactors as well as for downflow reactors for FCC processes. However, no document as such nor combined to other documents describes or suggests the inventive arrangement of a regenerated catalyst riser, a catalyst distributor provided with a perforated plate to allow optimized radial catalyst distribution, feed injectors to allow complete vaporization of the feed and a downflow reactor with a quick separation device inside a collector vessel provided with diplegless cyclones adapted for downflow reactor systems, such apparatus and FCC process being described and claimed in the present application.